The present invention relates to apparatus and methods for improving the reliability of high explosive devices utilizing detonation transmitting devices, such as detonating cords, and adapted for use downhole in a well.
High explosive devices are utilized for various purposes in wells, for example, to perforate the well casing. Such devices typically employ a number of high explosive charges joined by a detonating cord for group actuation. Often a succession of detonating cords will be run several hundreds of feet in order to permit several perforating guns to be detonated as a group and at widely spaced locations. Such operations are time consuming and expensive to carry out, and especially so where long or widely spaced intervals are to be perforated. It is, therefore, essential that the explosive devices operate reliably.
An advantageous well completion technique employs perforating guns lowered into the well on a tubing string. When the guns have been positioned adjacent the zones to be perforated, a packer is set to isolate the casing annulus adjacent the zones to be completed, the desired pressure condition in the annulus is established (for example, an underbalanced pressure condition) and then a detonating bar is dropped through the tubing from the surface to impact on a firing head to initiate the detonation of the guns through the detonation of the detonating cord.
The downhole environment presents a number of complicating factors which can interfere with the proper operation of the firing system. For example, in a highly deviated well, the detonating bar can become stuck in the tubing before impacting on the firing head. Also, in very hot wells, the operation of the impact-sensitive initiator can be adversely affected by heat so that, even if the bar does impact on the firing head, no detonation occurs. Even where the initiator operates properly, the detonating cord may fail to detonate its entire length. This can occur due to a break in the cord or a failure of the detonation to transfer from one length of cord to the next. Where it is necessary to run very long lengths of detonating cord, it correspondingly becomes more likely that the cord will not detonate its entire length, in which event it will be necessary to pull the string and attempt to complete the unperforated zones by repeating the entire operation.